Progressive spectacle lenses (also called varifocal lenses, multifocal lenses etc.) are usually understood to be spectacle lenses having a different (smaller) power in the region through which a spectacle wearer views an object located at a greater distance—hereunder designated as a distance portion—than in the region (near portion) through which the spectacle wearer views a near object.
Located between the distance portion and the near portion is the so-called progressive zone in which the power of the spectacle lens continuously increases from that of the distance portion to that of the near portion. The magnitude of the power increase is also designated as the power addition.
As a rule the distance portion is located in the upper part of the spectacle lens and designed for viewing “to infinity”, whilst the near portion is located in the lower region and is particularly designed for reading. In spectacles for special applications—those for pilots or for monitor work stations are mentioned as examples—the distance and near portions may also be arranged differently and/or designed for other distances. Furthermore, it is possible for a plurality of near portions and/or distance portions and corresponding progression zones to be present.
With progressive spectacle lenses having a constant refractive index it is necessary for the increase of power between the distance portion and the near portion that the curvature of one or both surfaces continuously change from the distance portion to the near portion.
The surfaces of spectacle lenses are usually characterized by the so-called principal radii of curvature R1 and R2 at every point of the surface. (Sometimes also the principal curvatures k1=1/R1 and K2=1/R2 are stated instead of the principal radii of curvature). Together with the refractive index n of the glass material, the principal radii of curvature determine the parameters frequently used for an ophthalmologic characterization of a surface:Surface power=0.5·(n−1)·(1/R1+1/R2)Surface astigmatism=(n−1)·(1/R1−1/R2).
The surface power is the parameter via which an increase of the power from the distance portion to the near portion is achieved. The surface astigmatism (lucidly termed cylinder power) is a “disturbing property”, because an astigmatism—inasmuch as an eye does not have an innate astigmatism to be corrected—exceeding a value of about 0.5 dpt results in an image on the retina which is seen to be indistinct.
Although any change of the curvature of the surface which is needed to achieve a surface power increase without vision being “disturbed” by surface astigmatism can be attained relatively simply along a (plane or curved) meridian, considerable “intersections” of surfaces will result alongside this meridian, leading to a large surface astigmatism which more or less impairs the lens in regions alongside the mentioned meridian.
With a surface having a surface power increasing from the distance portion to the near portion it is therefore not possible for reasons of surface theory to “maintain” the regions alongside a meridian (that is free from astigmatism or exhibits a predetermined astigmatism) free from physiologically disturbing surface astigmatism (Minkwitz Proposition).
Because the curvature in the distance portion must not change, it is relatively simple to design the distance portion of the progressive surface in such manner that the distance portion has a very small surface astigmatism (<0.5 dpt) or even a surface astigmatism value of “0” within a large region. On the other hand, the “quality” of the shaping of the regions lateral to the transition region is of decisive importance for the acceptability of the spectacle lens to the spectacle wearer concerned.
The basic object in designing every progressive spectacle lens is therefore to shape the lateral regions within the transition zone, as well as the lateral regions of the near portion if necessary, so that the spectacle lens will be as acceptable as possible to the spectacle wearer—without any unacceptable impairment of the distance portion.
In order to achieve this basic object, the designing of a progressive spectacle lens surface contributing to power change has in the past started out from a meridian lying or curving in a plane—also designated as a principal meridian or a principal line—as a “backbone for the design of the surface”. This meridian extends centrally along the surface from top to bottom, and its course approximately follows the point of penetration of the viewing rays through the spectacle lens surface during a shifting of the gaze and, in particular, a lowering of the gaze. The principal curvatures of every point of this meridian have been chosen so that the desired increase of surface power (also designated as addition power) from the distance portion to the near portion is achieved. Starting out from this meridian, the lateral regions of the surface have then been (more or less) suitably calculated.
For the shaping of the lateral regions a number of solutions have become known. During the early days of the calculation of progressive spectacle lenses a purely surface theoretical optimization of exclusively the progressive surface was performed, in which the foremost concern was that of substantially reducing as far as possible the disturbing surface astigmatism, or “displacing” the surface astigmatism into the lateral lower regions of the spectacle lens
Typical of this manner of proceeding are the progressive surfaces for spectacle lenses known from U.S. Pat. No. 2,878,721 or DE-AS 20 44 639, in which—provided that the progressive surface is the front surface—the curvatures of intersections of the progressive surface with plane surfaces which extend horizontally (horizontal sections) or are perpendicular to the principal meridian (orthogonal sections) are conic sections or curves of higher order, the curvature of which increases in the distance portion and decreases in the near portion. The transition between the increase of curvature of the sections in the distance portion and the decrease in the near portion occurs in the progressive zone.
This manner of proceeding results in the power difference and therewith also the surface intersections being decreased in the lateral regions. However, it is of disadvantage that because of the decrease of the power difference in the lateral regions an orientation of view through the edge region is rendered difficult and—according to the design—very disturbing swaying effects are experienced when looking through the lateral regions. Furthermore, the surface power along horizontal sections changes relatively strongly.
For this reason it was proposed in DE-Patent 28 14 936 that the curvature in the distance portion or in the near portion be allowed to decrease or increase only in a strip on both sides of the principal meridian, and that the change of curvature be reversed outside this strip.
However, common to all these approaches is a purely surface-related optimization of the progressive surface. This purely surface-related approach can be found also in more recent patent publications, such as DE-C-42 38 067 or DE-C-43 42 234.
In the last-mentioned publication an approach is described in which certain conditions for the gradient of the surface astigmatism and the surface power must be maintained. Irrespective of the circumstance that these gradients are not at all defined, for example when the surface is described by means of cubic splines, this purely surface-related optimization also disregards certain physiological prerequisites.
Approaches made to optimize the progressive surface in a wearing position are described in EP-A-677 177, U.S. Pat. No. 4,606,622 and DE 196 12 284.
For calculating a progressive surface in the wearing position, a wearing situation is established. This relates either to a particular user for whom the individual parameters have been specially determined in the respective wearing situation and the progressive surface has been separately calculated and fabricated, or to average values as described for example in DIN 58 208 Part 2.
However, even the known spectacle lenses which have been calculated for the wearing position have a number of disadvantages:
Thus, with the progressive surfaces known from U.S. Pat. No. 4,606,622 peaks are present along the course of lines of equal surface astigmatism for the lines having a surface power value of 5 dpt and 7 dpt.
Possibly the reason for this may be that surface related and wearing position related approaches have been mixed in the calculation.
DE-A-196 12 284 deals with a decrease of the mean power in the near portion towards the periphery. This is an inadequate approach, because also in the distance portion the power should not increase excessively towards the periphery.